1. Field of the Invention
This invention relates to a dry-spun elastic fiber made of a polyurethane-urea spandex polymer which has its soft segments derived from a copolymer of tetrahydrofuran and 3-methyltetrahydrofuran. In particular, the invention concerns such a fiber prepared from a spandex polymer that was derived from an isocyanate-capped glycol of the copolymer which had been chain extended with a mixture of diamines. The fiber of the invention has an unusually desirable combination of power, elongation, heat-set efficiency and hot-wet creep.
2. Description of the Prior Art
Elastic fibers made from polyether-based polyurethane-urea spandex polymers are well known for their resistance to hydrolysis and desirable properties of stretch, recovery and power. Product Licensing Index, Research Disclosures, item 8810 (August 1971) discloses such a spandex fiber, in which the polyether soft segment is derived from copolymers of tetrahydrofuran ("THF") and 3-methyltetrahydrofuran ("3-MeTHF"). The hydroxyl-terminated copolyethers from which the spandex polymer is made contain 4 to 20 mole % 3-MeTHF of 650 to 4,500 (preferably 2,400 to 3,800) molecular weight. Specifically disclosed is a copolymer of 3,300 MW containing 14% 3-MeTHF. Also disclosed are (a) "capped glycols" of 4,000 to 12,000 molecular weight, (b) diisocyanate capping agents, 2,4-tolylene diisocyanate, p,p'-methylenediphenyl diisocyanate, and 4,4'-methylenedicyclohexyl diisocyanate and (c) one or more conventional diamine chain extension agents, (e.g., ethylenediamine, propylenediamine and xylylenediamine. The thusly prepared segmented polymers contain 10 to 30% hard segments and are conveniently prepared in solvents such as dimethylformamide or dimethylacetamide for wet or dry spinning. The copolyether spandex fibers are stated to possess low hysteresis (i.e., small difference between stretch and recovery forces), good resistance to hydrolysis, and an improved combination of elongation and tenacity, as compared to similar fibers prepared from THF homopolymer.
Yarns made from the polymers specifically disclosed in the Product Licensing Index item discussed in the preceding paragraph, suffer from poor heat-setting characteristics which cause problems in the heat setting of certain fabrics and yarns which include the spandex fibers. The spandex fibers have a low heat-set efficiency. Yarns made with the fibers require long times and high temperatures for heat setting. In fabrics which include nylon fibers as well as such spandex fibers, high heat setting temperatures often lead to nonuniformities in the fabrics. Such yarns also often have low break strength, which can lead to difficulties in textile processing of the yarns into fabrics or to premature failure of the fabric during wear. Such difficulties and failures would be especially apparent with yarns having fine denier spandex filaments, such as are used in women's hosiery and swim suits. Thus, there is a need in the trade for such polyether-based spandex yarns that, in addition to their known advantageous characteristics, also possess good heat-setting properties at lower temperatures.